1. Field of the Invention
The present invention relates to an insulating state detection unit that detects an insulating state of an ungrounded power supply from a ground potential by using a flying capacitor, and particularly, relates to an insulating state detection unit having a failure detector that detects a failure of a switch switching a connection state of the flying capacitor to a charge/discharge circuit.
2. Description of the Related Art
In a vehicle using electric power as driving energy, it is usual that a DC power supply in which a voltage is boosted (for example, to 200V) is used as an ungrounded power supply by being insulated from a vehicle body. In the ungrounded power supply as described above, for detection of an insulating state of the ungrounded power supply from a ground potential, such as detection as to whether or not there is a ground fault, an insulating state detection unit using a flying capacitor is used.
The insulating state detection unit can charge, while switching internal switches thereof, the flying capacitor individually with electric charge amount corresponding to an output voltage of the DC power supply insulated from the ground potential, with electric charge amount corresponding to a ground fault resistance on a positive terminal side of the DC power supply, and with electric charge amount corresponding to a ground fault resistance on a negative terminal side of the DC power supply. Then, in a controller of the insulating state detection unit, respective charge voltages are measured, and based on measurement values thereof, the ground fault resistances on the positive terminal side and the negative terminal side are calculated, whereby the insulating state of the ungrounded power supply can be detected.
However, when a dead short circuit or a local short circuit occurs in the switches, the insulating state of the ungrounded power supply cannot be accurately detected. Accordingly, it has been proposed that the switches which should originally be switched on at the time of detecting the insulating state of the ungrounded power supply is switched to an OFF state one by one, and a state that occurs only when the switches switched to the OFF state are in an ON state is detected from a charge state of the flying capacitor, whereby a failure of each of the switches is detected.
Specifically, in a case where the flying capacitor, which is situated in a state of not discharging originally, discharges actually, it is determined that any of the switches present on a discharge circuit of the flying capacitor fails (refer to JP 3890503 B, JP 3890504 B, JP 3962991 B, and JP 2008-089323 A (patent family: U.S. Pat. No. 7,443,155 B2)).